1. Field of the Invention
The present invention relates to the quenching of avalanche currents in photodiodes. More particularly, the present invention relates to a method and apparatus for providing non-linear, passive quenching of avalanche currents in Geiger-mode avalanche photodiodes.
2. Related Art
In many applications in the optoelectronic arts, it is often beneficial to detect individual photons with an optoelectronic device. Single-photon detection can be accomplished using devices such as photomultiplier tubes and avalanche photodiodes (APDs). Photon counting with APDs is typically accomplished by operating the APD in the so-called “Geiger” mode, wherein the APD is biased above its zero-frequency breakdown voltage to produce an average internal gain on the order of one million or higher. Under such conditions, a readily-detectable avalanche current can be produced in response to a single input photon, thereby allowing the APD to be utilized to detect individual photons.
When a current avalanche is triggered in a Geiger-mode APD in response to a single input photon, the avalanche current continues as long as the bias voltage remains above the breakdown voltage of the APD. Thus, in order to detect the next photon, the avalanche current must be “quenched” and the APD reset. Quenching the avalanche current and resetting the APD involves a two-step process, wherein the APD bias is reduced below the APD breakdown voltage to quench the avalanche current as rapidly as possible, and the APD bias is then raised to a voltage above the APD breakdown voltage so that the next photon can be detected. During this process, the APD is incapable of detecting photons, thereby resulting in a “dead” time period. Therefore, it is beneficial to quench the avalanche current and reset the APD as quickly as possible to reduce dead time. Additionally, to minimize increases in the dark count rate (“after-pulsing”) that can occur with high photon arrival rates, it is also beneficial to limit the avalanche current to a minimum.
Various passive and active circuits have in the past been developed for quenching avalanche currents generated by Geiger-mode APDs. For example, the most basic passive quenching circuit is a resistor connected in series with a high-voltage bias applied to an APD. While such a circuit has the advantage of simplicity, this circuit typically results in detector dead times of many tens of microseconds. Due to the long resistor-capacitor (RC) time constant of this circuit, the bias across the APD varies continuously during the reset time, which results in undesirable variations in the photon detection probability of the APD. Various active quenching circuits overcome these limitations by employing a fast transistor circuit with numerous transistors to switch the bias voltage of the APD rapidly between voltages above and below the APD breakdown voltage. However, such circuits are often complex, require numerous components in addition to the transistors, and cannot easily be integrated into large photon-counting arrays.
Accordingly, what would be desirable, but has not yet been provided, is a simple and effective method and apparatus for quenching of avalanche currents in Geiger-mode APDs, wherein avalanche currents are rapidly quenched, detector dead time is minimized, and accurate photon detection is provided using a small number of components.